I. Field of the Invention
The present invention relates to the field of organic and inorganic thin films.
II. Related Art
Single crystals of high perfection and purity are desirable when studying the intrinsic properties of solid-state substances (Karl, 1980; Lawson and Nielsen, 1958). This has been achieved for many inorganic materials, such as Si and GaAs. In fact, the modem electronics industry benefited substantially from an enormous effort toward the growth and purification of silicon single crystals. In addition, our understanding of the fundamental properties of a solid such as structural, electronic, and optical properties depends strongly on the crystal quality. Progress in this area for organic molecular crystals has been limited mainly due to certain intrinsic characteristics of the molecules and their constituent solids. For example, the large spacing and weak interactions among neighboring molecules in a crystal make it relatively easy for foreign molecules to be incorporated into the lattice, often leading to an impure crystal with many defects (Wright, 1995).
A high-temperature annealing process to improve the crystal quality and purity (by allowing impurities to diffuse out), which is effective for inorganic solids, is not available for most organic crystals because of their low melting points and poor thermal stability. On the other hand, organic molecules generally have strong optical absorption coefficients, so thin films (of micrometer thickness) are required for characterization of their optoelectronic properties. The orientational dependence of light absorption of many molecules can only be studied with single crystals. To our knowledge, no externally controllable technique for the growth of single-crystal, micrometer thick films exists, although ordered ultrathin films (˜nm) can be prepared by Langmuir Blodgett deposition (Treggold, 1994), from self-assembled mono- and multilayers in solution, and more recently by organic molecular beam epitaxy in an ultrahigh vacuum chamber (Forrest, 1997).
The inventors previously demonstrated that several organic single-crystal thin-films can be prepared in sandwich cells made of two pieces of indium-tin oxide (ITO)-coated glass spaced about 1-2 μm apart, by capillary filling of the molten organic compound (Liu and Bard, 1999; Gregg et al., 1990). Examples of the organic compound are porphyrin, (Liu et al., 1996), sudan I (Liu et al., 1997) and solvent green 3 (Saito et al., 1997). However, many other materials form only amorphous or polycrystalline films when using the same procedure. Moreover, the purity of most organic films does not approach that characteristic of inorganic solid-state electronic materials.
It is believed that no other technique exists for growing a single-crystal film from an amorphous or microcrystalline film on a substrate such as glass or ITO. The technique of organic molecular beam epitaxy is performed in an ultrahigh vacuum chamber on a single crystal substrate, making it expensive and of limited usefulness (Forrest 1997). Lattice matching is a prerequisite in molecular beam epitaxy and an ordered structure can only be extended to a limited number of molecular layers to produce films of nanometer thickness.
For bulk organic materials and many inorganic materials, methods such as zone melting and zone refining are known wherein the purity and crystallinity of powders contained in tubes or other bulk configurations of organic substances is increased. Materials can also be purified by sublimation. However, the crystallinity of tube-processed or sublimed material will be reduced upon processing into a thin film. A method for the in situ purification is necessary for thin films.